Blood coagulation is a process consisting of a complex interaction of various blood components which eventually gives rise to a fibrin network, or clot. Degradation of the fibrin network can be accomplished by activation of the zymogen plasminogen into plasmin. Plasmin is a serine protease which acts directly to degrade the fibrin network and thereby regulate the coagulation process. Conversion of plasminogen into plasmin is normally catalyzed in vivo by tissue-type plasminogen activator (t-PA), a fibrin-specific serine protease which is believed to be the physiological vascular activator of plasminogen. Urokinase-type plasminogen activator (u-PA) is another member of the class of plasminogen activators characterized as serine proteases. t-PA and u-PA are functionally and immunologically distinguishable.
t-PA normally circulates as a single polypeptide chain of M.sub.r .apprxeq.72,000 daltons, which is converted to a two-chain form by cleavage of a peptide bond between amino acids 275 (Arg) and 276 (Ile). The heavy chain of t-PA (two variants of M.sub.r 40,000 and 37,000) is derived from the amino-terminus, while the light chain (M.sub.r 33,000) is derived from the carboxy-terminal end of the t-PA molecule. This cleavage is catalyzed by trypsin or plasmin, and is accompanied by an increase in activity, as measured using synthetic substrates, and by an increase in fibrinolytic activity. Single-chain t-PA becomes active upon binding to fibrin, probably due to a conformational change in the activator induced by binding to fibrin. Cleavage to the two-chain form may be associated with rapid clearance of t-PA from the bloodstream, but conflicting reports on this have been published (see Wallen et al., Eur. J. Biochem. 132: 681-686, 1983), and the clearance mechanism is poorly understood.
A two-dimensional model of the potential precursor t-PA protein has been established (Ny et al., Proc. Natl. Acad. Sci. USA 81: 5355-5359, 1984). From this model, it was determined that the heavy chain contains two triple disulfide structures known as "kringles." Similar kringle structures also occur in prothrombin, plasminogen and urokinase, and are believed to be important for binding to fibrin (Ny et al., ibid.). The second kringle (K2) of t-PA is believed to have a higher affinity for fibrin than the first kringle (K1) (Ichinose, Takio and Fujikawa, Journal of Clinical Investigation 78;163-169, 1986; Verheijen et al., EMBO J. 5: 3525-3530, 1986).
In addition, the heavy chain of t-PA contains a "growth factor" domain, a triple disulfide-bonded structure which has homology to epidermal growth factor and to similar domains in protein C, factor VII, factor IX and factor X.
The heavy chain of t-PA also contains a "finger" domain that is homologous to the finger domains of fibronectin. Fibronectin exhibits a variety of biological activities, including fibrin binding; its fibrin-binding activity has been correlated to four or five of its nine finger domains.
The light chain of t-PA contains the active site for serine protease activity, which is highly homologous to the active sites of other serine proteases.
The precursor form of t-PA additionally comprises a pre-region followed downstream by a pro-region, which are collectively referred to as the "pre-pro" region. The pre-region contains a signal peptide which is important for secretion of t-PA by vascular endothelial cells (Ny et al., ibid.). The pre sequence is believed responsible for secretion of t-PA into the lumen of the endoplasmic reticulum, a necessary step in extracellular secretion. The pro sequence is believed to be cleaved from the t-PA molecule following transport from the endoplasmic reticulum to the Golgi apparatus.
The use of t-PA for fibrinolysis in animal and human subjects has highlighted several shortcomings of the native molecule. The half-life of t-PA in vivo has been shown to be as short as three minutes in humans (Nilsson et al., Scand. J. Haematol. 33: 49-53, 1984). Injected t-PA is rapidly cleared by the liver, and, within 30 minutes, most of the injected material is metabolized to low molecular weight forms. This short half-life may limit the effectiveness of t-PA as a thrombolytic agent by necessitating high dosages. Typically, native t-PA is administered at a dose of 30 to 150 mg per patient, and the low solubility of the protein necessitates prolonged infusion. Fuchs et al. (Blood 65: 539-544, 1985) concluded that infused t-PA is cleared by the liver in a process independent of the proteolytic site and that infused t-PA will not accumulate in the body; that is, the clearance mechanism cannot be saturated. Furthermore, doses of t-PA sufficient to lyse coronary thrombi are far larger than normal physiological levels, and may cause activation of plasminogen throughout the body, leading to systemic degradation of fibrinogen (Sherry, ibid.), which results in dangerous bleeding episodes. This systemic activity is apparently due to the low specificity of the two-chain form of the molecule.
Various workers have modified t-PA in attempts to enhance its clinical suitability. Rosa and Rosa (International Patent Application WO 86/01538) modified the Lys at position 277 of t-PA to stabilize the single-chain form of t-PA. Ile (277) t-PA produced in E. coli was found to be less active as a single-chain molecule, as compared to native t-PA. Wallen et al. (ibid.) postulated that this lysine residue may be responsible for proteolytic activity of single-chain t-PA. Heyneker and Vehar (published British Patent Application 2,173,804) disclose amino acid substitutions around the cleavage site of t-PA. A variant t-PA comprising Glu at position 275 was shown to have greater specific activity, as compared to native t-PA. This variant t-PA also formed fewer complexes with t-PA inhibitor. The single-chain form was also shown to have greater affinity for fibrin than the two-chain form. Robinson (WO 84/01786) used enzymatic means to remove carbohydrate side chains from t-PA to increase plasma half-life. Van Zonneveld et al. (Proc. Natl. Acad. Sci. USA 83: 4670-4674, 1986) disclose modified forms of t-PA wherein portions of the heavy chain have been deleted. Robinson et al. (EP 207,589 A1) disclose mutant forms of t-PA in which the growth factor domain has been deleted or otherwise modified. However, these variant forms of t-PA do not fully overcome the problems associated with the native protein.
There remains a need in the art for a plasminogen-activating protein with a long half-life and high specificity for fibrin. The present invention fulfills this need by providing novel derivatives of tissue plasminogen activator in which the growth factor domain has been structurally disrupted. The t-PA analogs described herein provide significant advantages over native t-PA as therapeutic fibrinolytic agents by permitting the use of much smaller doses, thus overcoming the problems of low solubility of native t-PA and permitting administration by injection rather than infusion. Through the use of recombinant DNA technology, a consistent and homogeneous source of these proteins is provided. The proteins can be utilized to lyse existing clots in heart attack and stroke victims and in others where the need to lyse or suppress the formation of fibrin matrices is therapeutically desirable.